The present invention relates to time of flight (TOF) measurement systems. More particularly, the present invention relates to a ground penetrating radar (GPR) system that is capable of providing A-scan images of subsurface targets using a synthetic aperture, end-fire array.
In the past, GPR has been used for a number of diverse applications, for example, geophysical applications such as mapping subsurface strata; locating toxic waste sites for remediation; and detection and location of unexploded subsurface ordnance.
GPR systems are similar to ordinary radar systems in that both measure target range (i.e., the distance from the radar system to an intended target, or portion thereof) by determining the amount of time it takes for electromagnetic (EM) radiation to travel from the radar to the intended target and then back to the radar. In practice, however, conventional GPR systems are inherently more complicated than ordinary radar systems due to some unique problems associated with transmitting and receiving EM radiation through a subsurface medium.
The first problem is that the subsurface medium (e.g., the earth) is typically inhomogeneous. Therefore, the EM radiation in a GPR system must travel through a number of different media, for example, air, rock, sand, water, clay, and other types of subsurface mineral deposits, each with a different and unquantified dielectric constant. Hence, the propagation velocity of the EM radiation from point to point within the subsurface volume may vary dramatically and is typically unknown without first performing a detailed, time-consuming analysis of the subsurface volume.
Ordinary radars do not encounter this problem because they transmit and receive EM radiation through "free space" (i.e., air) which is a homogeneous medium with a known dielectric constant. Because the dielectric constant of air is known, the propagation velocity of the EM radiation traveling through the air is known. Therefore, the computation of target range is quickly reduced to the task of multiplying the EM radiation time-of-flight (i.e., the round trip travel time between the radar and the target) by the propagation velocity of EM radiation through air.
The second problem associated with conventional GPR is that EM radiation does not penetrate the earth as easily as it penetrates the air. In fact, some media, such as wet clay or salt water, are so absorbent that EM radiation, at the frequency ranges relevant to GPR, cannot penetrate more than a few inches. The ability to penetrate a subsurface medium is highly dependent upon the frequency of the EM radiation. More specifically, low frequencies tend to achieve greater subsurface penetration. Unfortunately, lower frequencies also result in decreased target range resolution (i.e., target range accuracy). However, range resolution is also dependent upon bandwidth. For some time, GPR systems have been employing ultra-wideband techniques, especially ultra-wideband impulse techniques which, to a significant extent, improve a GPR's ability to penetrate a subsurface medium without sacrificing resolution.
Although the two above-identified problems are by no means the only problems that affect GPR performance, they are clearly two very significant problems. Consequently, there is a need to produce a GPR system that, despite the above-identified problems, can produce a subsurface image in real- or near real-time. Moreover, there is a need to provide such a system that is physically compact so that it can be utilized in a spatially limited area.
In order to produce subsurface target images that are relatively free of noise/clutter, it is necessary to suppress radar echoes associated with both coherent and non-coherent noise/clutter. Non-coherent noise/clutter can be suppressed by employing any number of coherent integration techniques that are well-known in the art. The suppression of coherent noise/clutter is generally more problematic because coherent integration techniques do not attenuate coherent signals whether those signals are associated with one or more sources of noise/clutter or an intended subsurface target. Coherent noise/clutter is especially problematic when the corresponding echoes exhibit the same or similar time delays as the echoes associated with the intended target, because the echoes associated with the coherent noise/clutter are likely to result in "stationary targets" that appear in the image and interfere with or occlude the signal(s) associated with an intended target.
Coherent noise/clutter can emanate from any number of internal and/or external sources. Examples of such internal sources include the radar antenna, the radar receiver, and the attenuators. An example of an external source that might generate coherent noise/clutter is an object that remains at 90.degree. off boresight throughout the course of the radar dwell. It is important to note that the radar echoes associated with these various internal and external, coherent noise/clutter sources tend to be strong (i.e., powerful) signals relative to the radar echoes associated with intended subsurface targets, because the only transmission medium between the radar antenna and these noise/clutter sources is air, which does not attenuate radar energy nearly as much as subsurface media.
Accordingly, it would also be desirable to provide a GPR system that is capable of generating an A-scan image of a subsurface target, using a synthetic aperture, end-fire array technique, that has been thoroughly purged of any "stationary targets" that were caused by coherent noise/clutter sources. It is further desirable to minimize the amount of processing time required to produce that image, despite any additional processing tasks that are required to suppress the coherent noise/clutter.